Karl Mordo studied the magic arts under the Ancient One in Tibet when Dr. Stephen Strange came to study there. Strange foiled Baron Mordo's plot to kill the Ancient One, leading to Baron Mordo being cast out and Dr. Strange eventually becoming "Sorcerer Supreme." Mordo would clash with Dr. Strange on a multitude of occasions, at times with the backing of the demon Dormammu, going so far as successfully impersonating Dr. Strange for a short period of time.

A Transylvanian nobleman (born in Varf Mandra), Karl Amadeus Mordo became a student of the Tibetansorcerer known as the Ancient One.[2] When Mordo plotted to kill his teacher, Dr. Stephen Strange learned of the plot. Mordo was forced to cast restraining spells to prevent Strange from warning the Ancient One. In desperation, Strange decided to accept the Ancient One's offer to take him on as his mentor in magic to have some hope of stopping Mordo. The Ancient One, however, was fully aware of Mordo's plot and of Strange's desire to warn him. Pleased at Strange's unselfish decision, the Ancient One's magical teachings immediately freed the doctor and explained the full situation. Thus informed in the face of his recent experiences, Strange agreed to become the Ancient One's apprentice despite the requirement that he abandon his former life. The Ancient One trained him to be a formidable opponent of Mordo. Eventually Mordo was exiled by the Ancient One.[3] Mordo's abilities were similar to those of Doctor Strange, but Mordo was particularly skilled at astral projection and hypnosis, as well as mesmerism. He was more than willing to use powerful black magic and invoke demons, both of which Strange was reluctant or unable to do. Mordo's use of these darker arts would sometimes backfire.

The evil Mordo became an open foe of Doctor Strange. He sent his astral form to hypnotize the Ancient One's servant into poisoning his food, hoping the old man would reveal his secrets of magic. However, Strange tricked him into returning to his physical body after engaging in astral contact with him, thus breaking his hold over the servant, who helped the Ancient One to recover. Mordo disguised himself as Sir Clive Bentley, and trapped Doctor Strange with a drugged candle that paralyzed him, but Strange was able to use his mental powers to call a local girl to free him.[4] He opposed Doctor Strange's discipleship to the Ancient One.[5] He set a series of new traps for Strange in a bid to wrest magical secrets from the Ancient One.[6] Mordo took Strange's body while Strange was astral projecting for a threat he sensed that Mordo had caused, and cast a barrier around it, knowing that if his astral form was out for 24 hours it would die. Strange found it in a wax museum, but was unable to enter. However, Strange took control of a wax model in the last minute, distracting Mordo and enabling Strange to return to his body.[7] He imprisoned the Ancient One, but was again defeated.[8] Mordo made a deal with his new master, the demonic Dormammu of the Dark Dimension, to amass additional power to defeat Doctor Strange.[9] He dispatched agents, other evil magicians around the World and Spirit wraiths, to search for the incognito Strange,[10] and then banished him from the Earth.[11] He fled from Strange,[12] and then dispatched agents to kill Strange.[13] He engaged in personal combat with Strange.[14] Mordo was exiled to the Dimension of Demons by Dormammu.[15] He battled Doctor Strange once more at Stonehenge, but was ultimately banished from Earth.[16]

Mordo would return eventually to continue to bedevil Doctor Strange. He impersonated Doctor Strange during Strange's brief retirement, but was soon vanquished.[17] Mordo discovered the Book of Cagliostro, and battled Strange in 18th-Century Paris. He then accompanied Strange and Sise-Neg to the dawn of time.[18] After suffering a mental breakdown, he was placed in Doctor Strange's care, but later escaped.[19] Mordo transformed the dead Lord Phyffe into Azrael, angel of death, and dispatched him against Strange.[20] He sent the Man-Thing to kill Strange, and assembled thirteen people for human sacrifice to the Chaos Demon. Mordo was defeated by Strange, Jennifer Kale, and the Man-Thing.[21] He attacked Strange, and escaped into the 1940s, but was manipulated by Dormammu.[22]

Mordo later sold his soul to both Mephisto and Satannish for power, gaming that Strange would save him. He was trapped with Sara Wolfe, and later rescued her.[23] Mordo himself was later imprisoned, and Sara Wolfe freed him from imprisonment. Mordo battled Dormammu to defend the Earth. He was defeated, and pretended to ally himself with Dormammu, and then allied himself with Umar to defeat Dormammu.[24]

Umar and Mordo were eventually deposed by Clea.[25] Mordo eventually contracted terminal cancer as a side effect of his use of black magic, and renounced evil just before his death.[26] He later returned to life.[27] He later allies himself with Terrax, Tiger Shark and Red Hulk as the "Offenders", to be opposed by his old rival and newly costumed Doctor Strange, aided by the other three original Defenders.[28]

Mordo appeared in the pages of X-Factor Vol. 3 #203, where it is revealed that he kidnapped politician Cartier St. Croix in order to lure his daughter Monet St. Croix into a trap.[29]

Mordo appears to be deceased in Uncanny Avengers #6.

Mordo returns during the aftermath of the Last Days of Magic arc.[30] Mordo forces a family to leave their residence, but lets the mother stay as his servant. Dormammu incinerates the woman's body as he possesses her in order to chastise Mordo for delaying his plans. Mordo heads to the Sanctum Sanctorum and attacks Wong with Doctor Strange confronting him and the two briefly battle before the latter is taken by Nightmare.[31] Mordo, angered that his foe was taken by another, is confronted by Dormammu again (in the form of a swarm of rats) who proceeds to attack Mordo causing him to leave.[32] He later catches up to Doctor Strange when he is attacked by Orb, until Dormammu arrives. He then helps Doctor Strange in banishing Dormammu directly to Shuma-Gorath.[33]

During the "Secret Empire" storyline, Baron Mordo becomes the caretaker of Manhattan after it gets surrounded by Darkforce following Hydra's takeover of the United States. In addition to using the Sanctum Sanctorum as his residence, Baron Mordo has the Elder God Pluorrg guarding the Sanctum Sanctorum.[34] He later catches Daredevil, Luke Cage, Cloak and Iron Fist when they are attempting to fight him. Mordo is then defeated by Doctor Strange, Spider-Woman, Ben Urich and Kingpin.[35]

Baron Mordo has vast magical abilities derived from his years of studying black magic and the mystic arts. He can manipulate magical forces for a variety of effects, including hypnotism, mesmerism, thought-casting, and illusion casting. He can separate his astral form from his body, allowing him to become intangible and invisible to most beings. He can project deadly force blasts using magic, can teleport inter-dimensionally, and can manipulate many forms of magical energy. He can tap extra-dimensional energy by invoking entities or objects of power existing in dimensions tangential to Earth's through the recitation of spells. He can also summon demons, but often does not have enough power to force them to do what he wants them to do.

Baron Mordo has some knowledge of a karate-like martial art form, and has an extensive knowledge of magical lore.

Baron Mordo appears in the last issue of Mutant X, being referred to as the 'Ancient One'. He is still considered a villain, though he allies himself with other heroes and villains in order to stop the Beyonder/Goblin Queen entity.

In Secret Wars, the Avengers 2099 investigate the threat of the mysterious Martin Hargood.[36] In the final issue, it is revealed he is Baron Mordo's descendant and, after reclaiming his ancestral title, uses Alchemax's Virtual Unreality lab to summon the Dweller in Darkness. Captain America 2099 punches Mordo out, while Roman, the son of Namor, summons a Giganto to defeat the Dweller.[37]

Baron Mordo first appears in the Spider-Man: The Animated Series, voiced by Tony Jay.[38] In the show, Mordo appears as Dormammu's minion, and archenemy to Doctor Strange. In his first appearance in the episode "Sins of the Fathers Chapter 1: Dr. Strange", he was ordered by Dormammu to steal the Wand of Watoomb to make a portal that would release him from his dimension. To do so, Mordo brainwashes Mary Jane Watson to become his servant. His plan almost succeeded when Spider-Man, Dr. Strange, his assistant Wong, and Mary Jane stopped him from freeing Mordo's master from his dimension. Later, in "Venom Returns" and "Carnage", Mordo was ordered by Dormammu to bring the symbiote back to Earth so that Eddie Brock could once again bond with it and become Venom. Dormammu then orders Mordo to give Brock a message. At Stark Enterprises, Mordo disguises himself as "Ardon Broom" (an anagram of his true name) to get the portal. When Spider-Man and War Machine join forces to fight Venom, Dormammu tells Mordo that there is another symbiote, which Mordo then bestows to Cletus Kasady, naming the result "Carnage." After retrieving the probe, Dormammu orders Carnage to steal the "life force" energy from humans, so he may gain enough strength to enter our world. When Carnage absorbs too much force, Mordo brings him back to his lair to drain the power into an urn. When Carnage attempts to send the urn into Dormammu's portal, Spider-Man, Iron Man, and Venom interrupt him. Mordo ordered Carnage to throw the urn to unleash Dormammu, but was thwarted. Sacrificing himself, Venom pulled Carnage into the dimension following Dormammu's reversion, after which Mordo made his escape.

Baron Mordo appears in The Super Hero Squad Show, voiced by Dave Boat.[39] This version of the character is also completely insane. In the episode "Night in the Sanctorum", he was shown imprisoned in a soda can when Enchantress' magic vexes Thor to open the soda can containing Mordo. While belching out bubbles, Mordo ends up fighting Thor destroying Doctor Strange's kitchen until Doctor Strange appears and reimprisons Mordo in the soda can. In "Invader from the Dark Dimension", the soda can Mordo was imprisoned in was recycled into the Dark Dimension by Wong. When Iron Man ends up in the Dark Dimension through a portal near the statue of the Mayor of Superhero City, he encounters Mordo. Mordo manages to possess Iron Man, and uses his body to try to claim all of the Infinity Fractals. Upon turning Iron Man into the Iron Menace upon possessing him, Mordo takes over Stark Industries and uses mind control technology to take over Wolverine, Falcon, Redwing, and Abomination. Thor, Silver Surfer, and Hulk had to team up with Doctor Strange and Valkyrie to stop him even gaining some help from Doctor Doom and MODOK. Doctor Strange managed to trick Mordo back into the Dark Dimension and free Iron Man from his control.

Baron Mordo appears in the Ultimate Spider-Man, voiced by Danny Jacobs.[40] In the episode "Miles from Home", he was enlisted by Doctor Octopus to bring the Green Goblin of Earth-1610 to his reality since he was unable to transform his Norman Osborn back into Green Goblin. Even though the plan worked, Green Goblin of Earth-1610 attacked both of them and claimed the Siege Perilous. With help from Spider-Man, Doctor Strange engaged Baron Mordo in battle. After Spider-Man got Miles Morales to his world to help fight Green Goblin of Earth-1610, Baron Mordo and Doctor Octopus worked to reclaim the Siege Perilous from Green Goblin of Earth-1610 whose punches started opening holes in the fabric of reality. Green Goblin of Earth-1610 punched Baron Mordo causing a hole to open on him long enough to shatter him like glass as his remains fade away. In the episode "Strange Halloween," Baron Mordo had managed to reassemble himself in the Quantum Realm where he started using his magic in order to turn every trick-or-treater into monstrous versions of their costumes. Upon tracing Baron Mordo to the Quantum Realm, Spider-Man, Doctor Strange, and Ant-Man arrive where they face Baron Mordo and illusion versions of Doctor Octopus' nanite-powered form, Dormammu, and Ultron. Thanks to a tactic, Spider-Man, Doctor Strange, and Ant-Man shatter the illusions and unknowingly bring Baron Mordo back to their reality. Once there, Baron Mordo unleashed the transformed trick-or-treaters on the heroes and even animated the vehicles and buildings into monsters that obey his every command. Upon knowing of the imagination part of the spell, Spider-Man imagines the Eye of Agamotto enlarging which he uses to help Doctor Strange defeat Baron Mordo and undo his spell.

Baron Mordo appears in the Avengers: Secret Wars episode "The Eye of Agamotto" Pt. 1, voiced by Phil LaMarr.[41] This version is depicted to be a magic ally of Hydra. After some Hydra Agents raid a warehouse filled with magical items and steal the Talisman of Kaluu, Captain America and Black Panther track them to a building in Kaluu where they find Baron Mordo working on it. After a brief fight, Baron Mordo escapes as Captain America and Black Panther track him to an island where light and dark magic meet. With help from Shuri, Captain America and Black Panther defeat Baron Mordo who is dragged into another dimension with the portal being closed by a disguised Agamotto who destroys the Talisman of Kaluu.

Baron Mordo appears in the 2007 animated film Doctor Strange: The Sorcerer Supreme voiced by Kevin Michael Richardson. His origin is altered, yet maintains his dislike for Strange and the Ancient One. In this adaptation, he is a warrior-like sorcerer who began to think only about victory instead of what is truly important about their battles, willing to go as far as killing innocent children, which was against all the Ancient One stood for; this caused the Ancient One to reject his attempt to be Sorcerer Supreme. Believing the Ancient One has turned his back on him, Mordo sided with Dormammu and killed his former master. Doctor Strange, accepting his destiny as the new Sorcerer Supreme, managed to avenge the Ancient One and easily defeated Mordo. Angered at his failure, Dormammu eats his failed servant.

Chiwetel Ejiofor plays Mordo in the 2016 film Doctor Strange.[42] In the film, Karl Mordo is a member of the Masters of Mystic Arts who trains Strange, using the Staff of the Living Tribunal as his weapon. He helps Strange fight against Kaecilius. The Ancient One notes that Mordo's strength must be balanced by Strange since Mordo is unable to recognize the need for moral flexibility and compromise. At the end of the film, disillusioned from the teachings of the Ancient One after learning that the latter's immortality has been the result of her drawing on the energy of the Dark Dimension and Strange further breaking nature's laws, he decides to leave his fellow sorcerers. He comes to believe that all magic perverts and disrupts the natural order, thus causing him to set out to stop others from using it. In a post-credits scene, Mordo visits a fellow magician named Jonathan Pangborn and steals his energy that he uses to walk and make use of his legs, stating that Earth has "too many sorcerers."

Baron Mordo appears in the video game Marvel: Ultimate Alliance voiced by Philip Proctor. He is a member of the game's version of the Masters of Evil and one of Doctor Doom's lieutenants. He uses a spell to have the Orb of Teleportation transport the heroes to Murderworld instead of Castle Doom. He and Ultron fight the heroes in the path to Spire's Ascent after Doctor Doom and Loki left them behind. A simulation disk has Doctor Strange protecting Clea from Baron Mordo while in Mephisto's Realm. Another simulation disk has Ghost Rider protecting Wolverine from Baron Mordo while in Mephisto's Realm also. He does not speak in either simulation disk, though.

Baron Mordo is a playable character in the match-three mobile game Marvel Puzzle Quest. He was added to the game in February 2017.[44]

Baron Mordo appears as a playable character in Lego Marvel Super Heroes 2.[45] This version resembles the Marvel Cinematic Universe version. At the time when certain locations in time and space are used to make Chronopolis by Kang the Conqueror, Baron Mordo infiltrates the Sanctum Sanctorum to steal the Book of the Vishanti so that he can have Kang the Conqueror's Chronopolis be absorbed by the Dark Dimension. Doctor Strange, She-Hulk, and Spider-Man are able to defeat Baron Mordo and claim the Book of the Vishanti.[46]